Heater for thermal relays



Jun; 23, 1942. l. W. cox ETAL HEATER FOR THERMAL RELAYS Filed March 19, 1941 Patented June 23, 1942 HEATER FOR THERMAL RELAYS Irvin W. Cox, West Allis, and Edwin W. Seeger, Wauwatosa, Wis., assignors to Cutler-Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application March 19, 1941, Serial No. 384,036

5 Claims. (01. 201-67) This invention relates to improvements in thermal relays of ne type in which a heater coil whose temperature responds to the current of a translating device which is to be protected and which is connected in series with said heater coil affects a thermally responsive element. Upon responding to a given temperature, the thermally responsive element initiates the disconnection of the translating device from the power supply.

It is often desirable to group a number of translating devices, each provided. with a thermally responsive cutout, in parallel relation with each other, and to protect the entire group by a single fuse or other device of suficient capacity to carry the current of the entire group, and which has a correspondingly slow response.

The individual heater coils and their individdual translating circuits may therefore, in the event of a short circuit, be only protected by a device which has a capacity many times that of the respective heater coil. Hence if, on excessive sudden overload, an individual heater coil should burn out prior to the response of the respective thermal element associated therewith,

or if the latter should be short circuited, the arc which is established at the heating coil prior to the opening of the circuit by the group fuse produces a very large volume of gas and vapor which is highly conducting. If this vapor spreads into the ambient air it may cause short circuits between live parts of opposite polarity resulting in great damage.

For the aforementioned reason it is desirable to enclose the heater coil of the individual translating circuit in a substantially air-tight chamher to prevent the escape of highly conducting gases, but in doing so the pressure created inside of the enclosure becomes very high. It is therefore necessary to so construct the enclosure that it can withstand the pressure aforementioned without disruption, and that the arc, incident to the burning out of the heater coil, is permanently and safely extinguished or confined within said enclosure and without disruption of the latter.

An object of the present invention is to provide a heater for a thermal relay which upon burning out does not endanger the surrounding apparatus.

Another object is to provide a heater with eflicient thermal conductivity between the heating coil and a thermally responsive element aifected by the former.

Another object is to provide a heater operated thermal relay adapted to be used in series with one of a number of parallel connected translating i devices all of which are protected against excessive short circuit currents by a single fuse or circuit breaker.

Another object is to provide an enclosure for a heater for a thermally responsive element which can withstand high temperatures and high pressures without rupture.

Another object is to provide a heater structure which is thermally insulated from the ambient air, while at the same time providing for good heat conduction to the thermally responsive element.

Other objects and advantages will hereinafter appear.

The accompanying drawing illustrates an embodiment of the invention.

In the drawing,

Figure l is an irregular cross section of a heater embodying the invention-certain of the parts being shown in elevation.

Fig. 2 is a front elevation thereof, and

Fig. 3 is a diagram of connections of a group of parallel circuits embodying the heaters.

Referring to Figs. 1 and 2, the heater comprises a helical heater coil l. The ends 2 and 3 are bent toward one end of the helix parallel re1ation to the center line of the helix. The ends 2 and 3 are brazed, welded or otherwise fastened to the heads 4 and 5 of rivets 4 and 5 respectively, whose shanks pass through alined apertures in a head comprising a mica disc 6. a Bakelite disc "I and a steel disc 8. The walls of the two apertures in disc 8 are substantially larger than the diameter of the rivets and are spaced from the latter to electrically insulate said parts from each other (one of said apertures being shown at 3 in Fig. l). A cylinder 9, preferably of metal,

has one of its ends recessed at E0 to receive the disks 6, I and 8. At its other end the cylinder 8 has fastened to it by brazing or welding a head 12. The center of the head #2 is pressed. or spun iii-- wardly to form a well It, which extends inside of the helix i so as to be closely adjacent thereto. The space inside of well !3 is adapted to accommodate a thermally responsive element (shown in dotted lines at 33 in Fig. l.) to be con trolled by the heater.

To protect the inside of the cylinder 8 and the head I?! from arcs, they are lined with or covered by sheets of mica M and 15, respectively. The assembly of helix I with the discs 5, l and 8 after insertion thereof into recess ii! is retained by spinning the flange l! of cylinder I!) thereover. Thereafter a circular disc it of mica, having a diameter slightly smaller than that of the inturned flange II is positioned as shown in Figs. 1 and 2. The disc It is provided with openings to accommodate the shanks of rivets 4 and 5. A mica plate I! of rectangular shape having openings to accommodate said rivet shanks is interposed between the upper portion of flange I I, and terminal strips I8 and I9 of suitable shape and size to act as supporting brackets. The shank ends of rivets 4 and are soldered or otherwise mechanically and electrically connected to the respective brackets is and I9, as shown at 4 and 5 To prevent relative movement of the brackets I8 and I9 they are preferably further connected with the plate I! by means of rivets 26 and 2!, respectively.

In assembling the heater, the head I2 having the well I3 is assembled with the cylinder 9. Thereafter the mica elements I4 and I5 are positioned inside of the casing. The helix I having been welded to the rivets 4 and 5, and the discs 6, I and 8 having been positioned upon the rivet shanks, a mass of self-setting cement 22, is then spread over and about the helix I, and the inside surface of the mica disc 6. The discs 6, I and 3 and the parts associated therewith are then inserted into the casing and are held in assembled relation to the latter by spinning inwardly the end of cylinder 9 to form the flange II which lies against disc 8. The cement 22 hardens in place and protects the disc 6 from any are that may be established inside of the casing. The cement also spreads between the convolutions of the coil I and the outside surfaces of the well I3 within the casing so as to form an arc-protecting but heat-conducting layer between the coil I and the adjacent wall of the well I3. Thereafter the disc I6 and plate I! of mica are positioned as aforedescribed and the rivets 4 and 5 are soldered to the terminal strips I 8 and I9, respectively,the other steps in the assembling operation having been described above.

If an arc should occur inside of the casing it will be effectively confined therewithin, while the insulation about the helix I and over the inside walls of the casing will resist the attack of the are for a sufficiently long time to prevent the casing from burning through, and thus prevent conducting gases from reaching the outside of the casing. The heat resistance of the insulation inside of the casing and the heat capacity of the casing itself are suflicient to absorb the energy of the arc, until the protecting fuse or overload in series with the heater coil has opened the circuit.

Fig. 3 illustrates an application of the device. A number of translating circuits such as motors 30 are supplied with power from the alternating current lines L and L In series with each motor 30 is a heater coil 3| which upon overload aifects a switch 32 to open the circuit of the respective motor 30. All the motors with their heater coils are connected in parallel with each other and the entire group is connected in series with a fuse, circuit breaker, or other protecting device 34. Normally any overload of an individual motor will cause its heater coil SI and the switch 32 associated therewith to interrupt.

and disconnect the respective motor from the line. In the event of a sudden short circuit of considerable magnitude, such as indicated by the dotted line 35, the heater coil 3| of the respective motor would not immediately effect opening of the corresponding individual circuit switch 32, so that an excessive current would flow through the heater coil 3| before response of the circuit breaker 34. This is because the capacity of breaker 34 is high relatively to the current capacity of the individual heater coils 3|, wherefore response of the former is relatively slow, so

that the heater coil 3| may burn out, and establish an are inside of the enclosure. Such an arc, if not properly confined, would cause conducting gases to spread through the ambient air surrounding the heater coil 3|. This condition might lead to further short circuits. It is therefore necessary that the heater coil 3| be confined, so as to prevent the escape of conducting gases; and at the same time the confining structure must be of sufficient strength to withstand the increase in pressure of the enclosed air resulting from the heating of the latter by the arc.

We claim:

1. An electric heater for a thermally responsive device comprising in combination with a heater coil a metal tube surrounding said coil, an insulating head attached to one end of said tube provided with transverse passages for conductive portions at the ends of said coil to support said coil concentrically with respect to said tube and insulating the same therefrom, a metallic head attached to the other end of said tube and provided with a wall forming a central well extending into the inside of said coil, and an insulating and heat conducting plastic coating on said coil and covering at least a portion of the inside surface of said metal enclosure, said tube and said heads providing for a substantially air-tight enclosure of said coil.

2. An electric heater for a thermally responsive device comprising in combination with a heater coil, a substantially pressure-tight enclosure for said coil having relatively high thermal capacity, said enclosure comprising a metal tube surrounding said coil, an insulating head attached to one end of said tube provided with transverse passages for conductive portions at the ends of said coil to support said coil concentrically with respect to said tube and insulating the same therefrom, a metallic head attached to the other end of said tube and provided with a re-entrant wall forming a central well extending into the inside of said coil, an insulating and heat conducting coating on said coil and between the latter and said well and an insulating and heat resistant material covering the inside surfaces of said enclosure.

3. As an article of manufacture, a heater coil unit subjected to current of destructive magnitude comprising a substantially cup-shaped metal casing, the end wall of said casing having a re-entrant portion forming a well of substantially cylindrical contour to accommodate a thermally responsive element of suitable form, a heater coil positioned Within said casing and surrounding the wall of said well, for the purpose set forth, means for closing the open end of said casing, said last mentioned means being adapted to support said coil in electrically insulated relationship to said casing While providing for exposure exteriorly of said casing of conductive elements electrically connected to said coil, and a pair of terminal members mechanically and electrically connected with said conductive elements and arranged in insulated relationship to said metal casing, said terminal members being of bracket form to provide for attachment thereof to a suitable support whereby said unit may be supported in a predetermined fixed position.

4. As an article of manufacture, a heater coil unit subjected to currents of destructive mag nitude comprising a substantially cup-shaped metal casing, the end Wall of said casing having a re-entrant portion forming a well of substantially cylindrical contour to accommodate a thermally responsive element of suitable form, a heater coil positioned within said casing and surrounding the wall of said Well, for the purpose set forth, means for closing the open end of said casing, said last mentioned means being adapted to support said coil in electrically insulated relationship to said casing while providing for exposure exteriorly of said casing of conductive elements electrically connected to said coil, said last mentioned means including an insulating disc and a metal disc, the latter having openings formed therein to provide electrical clearance for said conductive elements, said open end of the casing having an integral annular flange bent inwardly over said discs to rigidly and permanently secure the latter in assembled position, and a pair of terminal members mechanically and electrically connected with said conductive elements and arranged in insulated relationship to said metal casing, said terminal members being of bracket form to provide for attachment thereof to a suitable support whereby said unit may be supported in a predetermined fixed position.

5. As an article of manufacture, a heater coil unit subjected to currents of destructive magnitude comprising a substantially cup-shaped metal casing, the end wall of said casing having a re-entrant portion forming a well of substantially cylindrical contour to accommodate a thermally responsive element of suitable form, a heater coil positioned within said casing and surrounding the wall of said well, for the purpose set forth, means for closing the open end of said casing, said last mentioned means being adapted to support said coil in electrically insulated relationship to said casing while providing for exposure exteriorly of said casing of conductive elements electrically connected to said coil, a pair of terminal members mechanically and electrically connected with said conductive elements and arranged in insulated relationship to said metal casing, said terminal members being of bracket form to provide for attachment thereof to a suitable support whereby said unit may be supported in a predetermined fixed position, means for electrically insulating said coil from the wall of the well within said casing, and means providing an insulating covering for substantially the entire area of all of the other inner walls of said casing.

IRVIN W. COX. EDWIN W. SEEGER. 

